This will, of course, void your warranty and invalidate your SmartNet contract. You have been warned.
Step 1: Find an Appropriate Dimm
It's been a while since such memory modules were prevalent in big box stores or even from OEMs, so if you do have them it's a good idea to test that they work in a less expensive piece of hardware beforehand. A lot can go wrong with memory that's been sloshing around in a box or static bag for years.
Step 2: Open Up Your Router . . .
Step 1 Turn off the router's power switch and disconnect the power supply
Step 2 Disconnect all cables from the router back panel.
Step 3 Use a Phillips-head screwdriver to remove the two screws on the rear of the unit (see above picture).
Step 4 Gently push the top of the router away from you and then lift it up from the bottom of the router.
Step 4 is a bit misleading. The cover of the router only comes off after you've disengaged a plastic clip on the front underside of the device. A flathead screwdriver helps. After this is done the top slides off easily.
Once the cover is off you'll find a thin-metal guard covering the internals. This is held in place by three phillips-head screws where the guard itself meets the PCB. Remove these and the guard comes right off, exposing an empty dimm slot.
Step 3: Install and Test Your Memory
1. Make sure the router is powered off and it's power brick is disconnected
2. Open the white retention clips on each side of the module seat
3. Align your memory module with the keyed plastic groove in the memory seat
4. Push down
5. The retention clips should swing up and hold the memory in place
Reconnect power and switch on your router. A 'show ver' from console will tell you if your memory has taken. An extended memory test afterwards is not a bad idea either.